> we just infer it from redshift. Not just redshift. We're using two observables: Redshift and brightness of a very specific kind of supernova which always rel

> we just infer it from redshift. Not just redshift. We're using two observables: Redshift and brightness of a very specific kind of supernova which always releases close to the same energy, i.e. amount of light. Using the 1/d² law we can relate observed brightness to distance. Redshift OTOH tells us the velocity. Then we plot "redshift vs. sqrt(1/brightness)" and if the expansion of the universe were constant, i.e. velocity/distance (i.e. the Hubble term being constant) we'd see a linear relation between distance and velocity. However what we actually observe is that the velocity is accelerating over distance, which means something is putting "something" extra into it. So far we can describe it only as a nonvanishing extra term in the field equations. A term which originally Einstein came up with to stabilize a static universe, but you can use it as well to "push" or "pull" on the whole of spacetime to accelerate/decelerate expansion/contraction. For all intents and purposes this term behaves like an additional energy term in the equations. We are in the _dark_ about, what's actually causing the effects we see. And when we adjust our models to accomodate for that, an extra _energy_ term shows up. Hence "dark energy". However no astrophysicist worth his salt thinks of it as something concrete. In the cosmology lectures I attended during my studies our professor used to pick some random name, who'd not be a student in the class at the time and instead use that, instead of "Dark Energy" to refer to that term (the "dark name" used when I was there was "Geroge" BTW), just to hammer the fact, that we don't have an effin clue what's going on, only that it's something very observable that's śhows up prominently in all the data collected for the past 20 years and you can't ignore it and have to give it some name.